The present invention relates to a feedforward distortion compensation circuit and, more particularly, to a feedforward distortion compensation circuit, for a linear amplifier, used in a high-frequency range.
For example, as described in Japanese Patent Laid-Open Nos. 1-198809 (Reference 1), 4-70203 (Reference 2), and 4-83406 (Reference 3), a conventional feedforward distortion compensation circuit of this type is used to realize a high-output linear amplifier used in a high-frequency range and to save power consumption.
FIG. 16 shows a basic arrangement of a conventional feedforward distortion compensation circuit.
A feedforward distortion compensation circuit is basically constituted by two signal canceling circuits. One of them is a distortion detection circuit 121, and the other is a distortion removal circuit 122. The distortion detection circuit 121 is constituted by a main amplifier path 123, a delay-line path 124, and a signal attenuation path 127. The distortion removal circuit 122 is constituted by a delay-line path 125, the signal attenuation path 127, and an auxiliary amplifier path 126. The main amplifier path 123 of the distortion detection circuit 121 is constituted by connecting a variable attenuator 131, a variable phase shifter 132, and a main amplifier 133 in series with each other, and the delay-line path 124 is constituted by a transmission line. The delay-line path 125 of the distortion removal circuit 122 is constituted by a transmission line, and the auxiliary amplifier path 126 is constituted by connecting a variable attenuator 136, a variable phase shifter 137, and an auxiliary amplifier 138 in series with each other. Reference numeral 128 denotes an input terminal to which an input signal is supplied, and reference numeral 129 denotes an output terminal from which an output signal is transmitted.
In this case, since characteristics do not considerably change, the delay-line path 124 may comprise one or both of the variable attenuator 131 and the variable phase shifter 132. Similarly, the delay-line path 125 may comprise one or both of the variable attenuator 136 and the variable phase shifter 137. The main amplifier path 123 of the distortion detection circuit 121 is connected to the delay-line path 125 of the distortion removal circuit 122 through a power distributer 134, and the delay-line path 124 of the distortion detection circuit 121 is connected to the auxiliary amplifier path 126 through a power synthesizer 135. An attenuator 139 is connected to the signal attenuation path 127 between the power distributer 134 and the power synthesizer 135 as needed. Reference numeral 130 denotes a power distributer for distributing an input signal input from the input terminal 128 to the main amplifier path 123 and the delay-line path 124, and reference numeral 140 denotes a power synthesizer for synthesizing a signal from the delay-line path 125 with a signal from the auxiliary amplifier path 126 to output the synthesized signal to the output terminal 129.
The operation of the above feedforward distortion compensation circuit will be described below.
An input signal supplied to the input terminal 128 is distributed to the main amplifier path 123 and the delay-line path 124 by the power distributer 130. A distorted signal component passing through the main amplifier path 123 is distributed to the delay-line path 125 and the signal attenuation path 127 by the power distributer 134. The distorted signal component passing through the signal attenuation path 127 is synthesized with a signal component passing through the delay-line path 124 in the power synthesizer 135, and the synthesized signal is output to the auxiliary amplifier path 126. At this time, the variable attenuator 131 and the variable phase shifter 132 are adjusted such that both the signal components passing through two paths input to the power synthesizer 135, i.e., the delay-line path 124 and the signal attenuation path 127 are equal to each other in amplitude and delay amount and respectively have opposite phases. In this case, the conditions for the opposite phases are realized by properly setting a phase shift amount between the input and output terminals of each of the power distributors 130 and 134 and the power synthesizer 135 or by using phase inversion in the main amplifier 133. Otherwise, as shown in FIG. 17 showing a phase inversion circuit using a circulator, when a phase inversion circuit in which one terminal of a circulator 141 is short-circuited and terminated by a terminator 142 is connected to the main amplifier path 123, the delay-line path 124, or the signal attenuation path 127, the above conditions can be realized.
Since the distortion detection circuit 121 is arranged as described above, a difference component between two signals passing through the delay-line path 124 and the signal attenuation path 127 is output from the power synthesizer 135 to the auxiliary amplifier path 126. More specifically, when the variable attenuator 131 and the variable phase shifter 132 are adjusted as described above, the difference component is constituted by only a nonlinear distortion component generated by the main amplifier 133. For this reason, the circuit 121 is called a distortion detection circuit.
Of signals distributed by the power distributer 134 and passing through the main amplifier path 123, a signal output to the delay-line path 125 is synthesized by the power synthesizer 140 with a signal passing through the auxiliary amplifier path 126. At this time, the variable attenuator 136 and the variable phase shifter 137 are adjusted such that the transfer characteristics of the two paths from the input terminal of the power distributer 134 to the output terminal 129, i.e., the delay-line path 125 and the pair of the signal attenuation path 127 and the auxiliary amplifier path 126, are equal to each other in amplitude and delay amount, and respectively have opposite phases. In this case, since a signal passing through the auxiliary amplifier path 126 is the distortion component of an output signal output from the main amplifier 133 and detected by the distortion detection circuit 121, the power synthesizer 140 synthesizes the output from the main amplifier 133 with the distortion component to respectively have opposite phases and equal amplitudes. As a result, the distortion components of an output from the overall circuit are canceled out.
The above operation is the basic operation of a general feedforward distortion compensation circuit. The distortion compensation performance of the feedforward distortion compensation circuit largely depends on the adjustment accuracy of the variable attenuator 131 and the variable phase shifter 132 in the distortion detection circuit 121 and the adjustment accuracy of the variable attenuator 136 and the variable phase shifter 137 in the distortion removal circuit 122. However, the above adjustment cannot easily performed such that a balanced state which satisfies the above conditions is kept. Even if perfect adjustment is performed by an initial control operation, the characteristics of the main amplifier 133 change due to a change in environmental temperature, a small change in power supply voltage, a change in input power, deterioration over time, or the like. For this reason, the transfer characteristics of the distortion detection circuit 121 and the distortion removal circuit 122 cannot be easily followed.
Therefore, for example, in the prior art described in Reference 3, as a means for accurately detecting the transfer characteristics, the following means is used. That is, a first pilot signal is input to the input terminal of the distortion compensation circuit at a specific frequency, and a second pilot signal is input to the main amplification path at a specific frequency. A residual first pilot signal level in the distortion detection circuit is detected in the auxiliary amplifier path, and a residual second pilot signal level in the distortion removal circuit is detected by an output from the distortion compensation circuit. The variable attenuator and variable phase shifter of the main amplifier path and the variable attenuator and variable phase shifter of the auxiliary amplifier path are adjusted such that the detection levels of these signals become minimum.
In a conventional feedforward distortion compensation circuit, a variable attenuator and a variable phase shifter are adjusted on the basis of only the detected residual pilot signal levels. However, since the residual pilot signal levels detected in this case do not directly represent the adjustment amounts of the variable attenuator and the variable phase shifter, control of the variable attenuator and the variable phase shifter inevitably depends on only an estimative algorithm. For this reason, not only an excessive load acts on a control circuit but also a large number of steps are required to cause the adjustment amounts to reach optimal adjustment amounts. For this reason, an abrupt change in characteristics or a small change in characteristics cannot be easily followed.